1. Field of the Invention
This invention relates generally to micro-electro-mechanical systems (MEMS) for energy harvesting and more particularly to an electromagnetic MEMS power generator for converting vibrational energy into electrical power.
2. Description of the Related Art
There is a growing interest in the field of miniature sensors in applications such as medical implants and embedded sensors in buildings. One of the projected goals for micro-electro-mechanical systems (MEMS) technology is to develop low-cost and high-performance distributed sensor systems for medical, automotive, manufacturing, robotics, and household applications. One area that has received little attention is how to effectively supply the required electrical power to such sensor elements. Many applications require the sensors to be completely embedded in a structure with no physical connection to the outside world. Ideally, the elements of these distributed systems have their own integrated power supplies to reduce problems related to interconnection, electronic noise and control system complexity. Efforts are underway to develop integrated chemically-based power supplies with MEMS devices. Chemical power supply (battery) technology is well-developed for such applications but, where shelf life or replacement accessibility is a limiting factor, chemical power supplies may not be suitable for the application. Another approach to supplying power to such systems is to include a renewable power supply within the sensor element, thereby making them self-powered Microsystems.
Renewable power supplies convert energy harvested from an existing energy source within the environment into electrical energy. The preferred source of energy depends on the application. Some possible energy sources include optical energy from ambient light such as sunlight, thermal energy harvested across a temperature gradient, volume flow energy harvested across a liquid or gas pressure gradient, and mechanical energy harvested from motion and vibration. Of these sources, light and thermal energy have already been exploited for use in micro-power supplies. However, there are many applications where there is an insufficient amount of light or thermal energy such as in medical implants. Therefore, practitioners in the art have proposed many different power supplies that generate electricity from ambient mechanical energy. Ambient mechanical vibrations inherent in the environment, from the movement of our bodies to the hum of a computer, can provide a constant power density of 10 to 50 μW/cc.
Several practitioners have proposed rudimentary vibration-based power generators at the University of Sheffield [C. B. Williams, R. B. Yates, “Analysis of a microelectric generator for microsystems,” 8th Intl. Conf. on Solid-State Sens. & Actutators, Stockholm, Sweden. 25-29 Jun. 1995, 87-B4, pp. 369-72] and Massachusetts Institute of Technology [Scott Meninger, Jose Oscar Mur-Miranda, Rajeevan Amirtharajah, Anantha P. Chandrakasan, and Jeffrey H. Lang, “Vibration-to-Electric Energy Conversion,” IEEE Trans. on VLSI Systems Vol. 9, No. 1, pp. 64-76, February 2001]. for example. Williams et al. describe a theoretical model of an electromagnetic micro-generator for harvesting vibrational energy by accumulating the current created by changes in magnetic flux at a copper coil induced by the vibration of a nearby permanent magnet. Meninger, et al. describe a micro-generator that harvests vibrational energy by accumulating the voltage created by vibration-induced changes in a variable capacitor.
Others have recently improved on the earlier efforts. For example, Ching et al. [Neil N. H. Ching, H. Y Wong, Wen J. Li, Philip H. W. Leong, and Zhiyu Wen, “A laser-micromachined multi-modal resonating power transducer for wireless sensing systems,” Sensors and Actuators A: Physical, Vol. 97-98, pp. 685-690, 2002.] describe a micromachined generator with enough power to drive an off-the-shelf circuit. For this work, Ching et al. prefer micromachining methods to build their vibration-induced power generator because the methods afford precise control of the mechanical resonance necessary for generator efficiency, and batch fabricability for low-cost mass production of commercially viable generators. Similarly, Williams et al. later describe [C. B. Williams, C. Shearwood, M. A. Harradine, P. H. Mellor, T. S. Birch and R. B. Yates, “Development of an electromagnetic micro-generator,” IEE Proc.—Circuits Devices Syst., Vol. 148, No. 6, pp. 337-342, December 2001] a simple inertial generator built according to their earlier theoretical analysis that is also fabricated by means of micromachining. Other examples include the laser-micromachined electromagnetic generator described by Li et al. [Wen J. Li, Terry C. H. Ho, Gordon M. H. Chan, Philip H. W. Leong and Hui Yung Wong, “Infrared Signal Transmission by a Laser-Micromachined Vibration-Induced Power Generator,” Proc. 43rd IEEE Midwest Symp. on Circuits and Systems, Lansing Mich., 08-11 Aug. 2000, pp 236-9], which provides 2 VDC power sufficient to send 140 ms pulse trains every minute when subjected to 250 micron vibrations in the 64-120 Hz region.
In U.S. Pat. No. 6,127,812, Ghezzo et al. describe an energy extractor that includes a capacitor that experiences capacitance and voltage changes in response to movement of a capacitor plate or of a dielectric material. In one embodiment, a third plate is positioned between first and second plates to create two capacitors of varying capacitances. In another embodiment, one capacitor plate is attached by flexible arms which permit movement across another capacitor plate. The above capacitors can be used singularly or with one or more other capacitors and are rectified either individually or in a cascaded arrangement for supplying power to a rechargeable energy source. The above capacitors can be fabricated on a substrate along with supporting electronics such as diodes. Ghezzo et al. employ varying capacitance and neither consider nor suggest any solution the problem of fabricating an electromagnetic micro-generator.
In U.S. Pat. No. 6,722,206 B2, Takeda describes a force sensing device having an element of magnetic material mounted to a substrate such that another magneto-electrical material element is subjected to the magnetic field generated by the magnetic member. A movable member is mounted for oscillation in response to vibration and such oscillation changes the magnetic field experienced by the magneto-electrical material, which in turn changes an electrical property of the magneto-electrical material. Takeda neither considers nor suggests any solution the problem of fabricating an electromagnetic micro-generator.
Despite the efforts of several practitioners in the art, there still exists a need in the art for an electromagnetic micro-generator suitable for inexpensive fabrication in volume at the MEMS scale that can generate power sufficient for operating today's microchips. The electromagnetic devices known in the art all generally employ a single magnetic mass which oscillates on a spring element to change the magnetic flux at a nearby stationary coil. These devices are thereby limited in power output capacity by the limited mass of the single magnet, the limited room for a number of coils in the flux field of the single magnet and the limited flux slope available at the coils because of the single magnetic pole exposed thereto. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.